Recent Progress in Metal Oxide-Based Photocatalysts for CO2 Reduction to Solar Fuels: A Review

One of the challenges in developing practical CO2 photoconversion catalysts is the design of materials with a low cost, high activity and good stability. In this paper, excellent photocatalysts based on TiO2, WO3, ZnO, Cu2O and CeO2 metal oxide materials, which are cost-effective, long-lasting, and easy to fabricate, are evaluated. The characteristics of the nanohybrid catalysts depend greatly on their architecture and design. Thus, we focus on outstanding materials that offer effective and practical solutions. Strategies to improve CO2 conversion efficiency are summarized, including heterojunction, ion doping, defects, sensitization and morphology control, which can inspire the future improvement in photochemistry. The capacity of CO2 adsorption is also pivotal, which varies with the morphological and electronic structures. Forms of 0D, 1D, 2D and 3DOM (zero/one/two-dimensional- and three-dimensional-ordered macroporous, respectively) are involved. Particularly, the several advantages of the 3DOM material make it an excellent candidate material for CO2 conversion. Hence, we explain its preparation method. Based on the discussion, new insights and prospects for designing high-efficient metallic oxide photocatalysts to reduce CO2 emissions are presented.


Introduction
According to the report from Carbon Emission Accounts and Datasets for emerging economies (CEADs), China has emitted more than 10 billion tons of CO 2 per year since 2018. It is inextricably linked to people's desire for a more comfortable and convenient life. Nearly 60% of total emissions emanated from the production of cement, steel and coal-fired power generation processes. As a result of the greenhouse effect caused by ever-growing CO 2 , the global average temperature is continuously increasing [1] and various types of severe weather will incur more frequently, such as ice melting, land drought, typhoons, hurricanes, earthquakes and tsunamis [2]. It is, therefore, imperative to convert and utilize the CO 2 present in the atmosphere.
The transformation of CO 2 can be driven by illumination, electricity and heat [3]. Solar energy is a safe, clean, renewable and inexhaustible energy source, so it is ingenious to achieve this conversion with sunlight [4]. Additionally, the reaction conditions in photocatalytic processes are mild [5]; thus, it is easy to conduct experimental tests. A new world opened up since H 2 and O 2 were obtained after radiating TiO 2 with light. Extensive research about photocatalyst were conducted until the year of 2000. Since then, a growing number of materials have been designed and studied to absorb solar energy, including oxide semiconductors, sulfides (ZnS, CdS and MoS 2 ), and polyoxometalates (Bi 2 WO 6 , Bi 2 MoO 6 and BiFeO 3 ) [6,7]. Organics, organometallic complexes, covalent organic polymers and noncovalent self-assembled supramolecular organic matter are also involved. Among them, inorganic metal oxide materials are widely studied for establishing efficient artificial photosystems due to their low cost, facile synthesis, stable crystal structures and environmental friendliness. TiO 2 , Cu 2 O, ZnO, WO 3 and CeO 2 show promising research value as the most common materials.
Despite the significant progress that has been achieved, researchers still have difficulties in developing highly active catalysts because of poor light-harvesting capacity, low CO 2 adsorption capacity and rapid recombination of charge carriers. To this end, theoretical foundation and strategies to upgrade photocatalysts are described in detail in this article. Firstly, the advantages of exposed facets adjustment, and the morphology of 0D, 1D, 2D and 3DOM materials are summarized. Then, common co-catalyst materials are introduced. Defects, ion doping and sensitization engineering are also discussed. We provide a basic understanding on these approaches to inspire the future improvement in photocatalytic field. Furthermore, as inorganic metal oxide materials are more suitable for large-scale production, this paper can serve as a model for future industrialization.

Theoretical Foundation and Strategies of Photocatalytic CO 2 Reduction
The photocatalytic process in semiconductors can generally be described as shown in Figure 1. Upon being excited by an incident photon with energy equal to or higher than the bandgap (Eg), charge carriers are generated. Electrons (e − ) at the bottom of conduction band (CB) migrate to the surface of the catalyst to initiate reduction reactions with CO 2 . Holes (h + ) at the top of valence band (VB) conduct oxidative reactions.
research about photocatalyst were conducted until the year of 2000. Since then, a growing number of materials have been designed and studied to absorb solar energy, including oxide semiconductors, sulfides (ZnS, CdS and MoS2), and polyoxometalates (Bi2WO6, Bi2MoO6 and BiFeO3) [6,7]. Organics, organometallic complexes, covalent organic polymers and noncovalent self-assembled supramolecular organic matter are also involved. Among them, inorganic metal oxide materials are widely studied for establishing efficient artificial photosystems due to their low cost, facile synthesis, stable crystal structures and environmental friendliness. TiO2, Cu2O, ZnO, WO3 and CeO2 show promising research value as the most common materials.
Despite the significant progress that has been achieved, researchers still have difficulties in developing highly active catalysts because of poor light-harvesting capacity, low CO2 adsorption capacity and rapid recombination of charge carriers. To this end, theoretical foundation and strategies to upgrade photocatalysts are described in detail in this article. Firstly, the advantages of exposed facets adjustment, and the morphology of 0D, 1D, 2D and 3DOM materials are summarized. Then, common co-catalyst materials are introduced. Defects, ion doping and sensitization engineering are also discussed. We provide a basic understanding on these approaches to inspire the future improvement in photocatalytic field. Furthermore, as inorganic metal oxide materials are more suitable for large-scale production, this paper can serve as a model for future industrialization.

Theoretical Foundation and Strategies of Photocatalytic CO2 Reduction
The photocatalytic process in semiconductors can generally be described as shown in Figure 1. Upon being excited by an incident photon with energy equal to or higher than the bandgap (Eg), charge carriers are generated. Electrons (e − ) at the bottom of conduction band (CB) migrate to the surface of the catalyst to initiate reduction reactions with CO2. Holes (h + ) at the top of valence band (VB) conduct oxidative reactions. The photocatalytic process of CO2 conversion on semiconductors can be divided into five general steps: (1) Formation of electron-hole pairs under light radiation, (2) Separation and migration of the electrons and positive holes, (3) Adsorption and activation of CO2, (4) Redox reactions that between surface-adsorbed species and electron-hole pairs, and (5) Desorption of the product. The electrochemical reactions with standard oxidation-reduction potentials (at pH 7 vs. NHE) are as follows: The photocatalytic process of CO 2 conversion on semiconductors can be divided into five general steps: (1) Formation of electron-hole pairs under light radiation, (2) Separation and migration of the electrons and positive holes, (3) Adsorption and activation of CO 2 , (4) Redox reactions that between surface-adsorbed species and electron-hole pairs, and (5) Desorption of the product. The electrochemical reactions with standard oxidationreduction potentials (at pH 7 vs. NHE) are as follows: To carry out the conversion, it is necessary to comprehend the structure of CO 2 . Due to the great symmetry and high bond energy of 750 kJ/mol, it is particularly difficult to break the C=O double bond. Therefore, the transmutation between CO 2 and bent radical anion of CO 2 •− on the surface of the catalyst is widely recognized as the first step to activate CO 2 for subsequent reaction. Additionally, photoreactions occur favorably only when the CB position of the catalyst presents a more negative potential than the target reduction and the VB position is more positive than the oxidation reaction. Figure 2A displays the CO 2 reduction potentials of some common semiconductors, along with the Eg positions. From a molecular perspective, the adsorbed CO 2 is combined with e − , H + or other intermediates on the surface of the catalyst. On the basis of newly-released reports, the photocatalytic mechanism of CO 2 reduction to CH 4 /CO is graphically explained in Figure 2B [8][9][10].
CO2 + 6H + + 6e − → CH3OH + H2O, E0 = −0.38 V CO2 + 8H + + 8e − → CH4 + 2H2O, E0 = −0.24 V To carry out the conversion, it is necessary to comprehend the structure of C to the great symmetry and high bond energy of 750 kJ/mol, it is particularly di break the C=O double bond. Therefore, the transmutation between CO2 and ben anion of CO2 •− on the surface of the catalyst is widely recognized as the first step to CO2 for subsequent reaction. Additionally, photoreactions occur favorably only w CB position of the catalyst presents a more negative potential than the target r and the VB position is more positive than the oxidation reaction. Figure 2A disp CO2 reduction potentials of some common semiconductors, along with the Eg p From a molecular perspective, the adsorbed CO2 is combined with e − , H + or oth mediates on the surface of the catalyst. On the basis of newly-released reports, th catalytic mechanism of CO2 reduction to CH4/CO is graphically explained in Figu  10]. Therefore, enough e − and H + are requisites for the entire photoreduction pr series of fuels can be obtained by means of well-designed catalysts, such as CH CH3OH [14] and HCOOH [15]. It makes sense to create valuable solar fuel fr which provides a solution to environmental pollution. Products, alcohols, hydro and even carbon monoxide can be used as feedstock for energy reserves or hi compounds. Figure 3 summarizes the current strategies that can boost the photocatalytic duction pace, with detailed information being presented sequentially below. Therefore, enough e − and H + are requisites for the entire photoreduction process. A series of fuels can be obtained by means of well-designed catalysts, such as CH 4 [12,13], CH 3 OH [14] and HCOOH [15]. It makes sense to create valuable solar fuel from CO 2 , which provides a solution to environmental pollution. Products, alcohols, hydrocarbons and even carbon monoxide can be used as feedstock for energy reserves or high-value compounds. Figure 3 summarizes the current strategies that can boost the photocatalytic CO 2 reduction pace, with detailed information being presented sequentially below.
Molecules 2023, 28, x FOR PEER REVIEW

Morphology Control
The surface topography of nanocrystals could evidently alter the electronic str surface energy and chemical properties of catalysts. Therefore, morphology contro of the most important issues that concerns researchers in nanoscience, chemis physics. Open facets and edges determine the shape of nanocrystals. Thus, the pref adsorption of additives on certain crystal surfaces provides a good opportunity the surface of nanomaterials.

Exposed Facet Adjustment
Exploring and figuring out the variation that is connected with exposed sur crucial to elucidate shape-related chemical and physical properties. In semicon crystals, different facets have distinct electronic band structures that influen transport of photoexcited carrier charges. It is wise to expose active facets to tune t photoreduction efficiency [16].

Quantum Dots (QDs)
Zero-dimensional (0D) semiconductor quantum dots (QDs) have many uniqu erties, such as quantum confinement effect, high extinction coefficient and multip ton generation [17]. Hence, QDs show much better photoactivity in the visible light Unlike bulk materials, surface atoms make up the majority of QD semiconducto abundant surface sites enhance the interaction between electron donors and acc thus facilitating the photocatalytic charge transfer rate.

One-Dimensional and Two-Dimensional Structures
One-dimensional nanostructured catalysts have high aspect ratios, such a owires, nanorods and nanotubes. The morphological tuning of the material make ference to their thermal, optical, electrical and magnetic properties [18]. For instan TiO2 nanotube can act as a channel for electron transfer and build up the chemic tions rate.
Two-dimensional layered materials can protect a tiny particle component fr gregating. The CO2 adsorption capacity on the surface of 2D photocatalysts can

Morphology Control
The surface topography of nanocrystals could evidently alter the electronic structure, surface energy and chemical properties of catalysts. Therefore, morphology control is one of the most important issues that concerns researchers in nanoscience, chemistry and physics. Open facets and edges determine the shape of nanocrystals. Thus, the preferential adsorption of additives on certain crystal surfaces provides a good opportunity to tune the surface of nanomaterials.

Exposed Facet Adjustment
Exploring and figuring out the variation that is connected with exposed surfaces is crucial to elucidate shape-related chemical and physical properties. In semiconductor crystals, different facets have distinct electronic band structures that influence the transport of photoexcited carrier charges. It is wise to expose active facets to tune the CO 2 photoreduction efficiency [16].

Quantum Dots (QDs)
Zero-dimensional (0D) semiconductor quantum dots (QDs) have many unique properties, such as quantum confinement effect, high extinction coefficient and multiple exciton generation [17]. Hence, QDs show much better photoactivity in the visible light region. Unlike bulk materials, surface atoms make up the majority of QD semiconductors. The abundant surface sites enhance the interaction between electron donors and acceptors, thus facilitating the photocatalytic charge transfer rate.

One-Dimensional and Two-Dimensional Structures
One-dimensional nanostructured catalysts have high aspect ratios, such as nanowires, nanorods and nanotubes. The morphological tuning of the material makes a difference to their thermal, optical, electrical and magnetic properties [18]. For instance, the TiO 2 nanotube can act as a channel for electron transfer and build up the chemical reactions rate.
Two-dimensional layered materials can protect a tiny particle component from aggregating. The CO 2 adsorption capacity on the surface of 2D photocatalysts can be enhanced due to the large specific surface area and bountiful surface defects [19]. Two-dimensional lamellar nanosheets are widely used in photocatalysts, such as g-C 3 N 4 , MoS 2 and WO 3 . Macroporous materials are being widely used in massive photocatalytic materials, owing to their excellent properties [20]. Unlike dispersed particles, sunlight can penetrate the pore wall easily and scatter widely inside the hollow structure, thus increasing the efficiency of illumination. Subsequently, the slender walls of pore reduce the transfer length of photo-generated charge carriers. Electrons (e − ) and holes (h + ) are separated more efficiently when heterojunctions are loaded on porous materials. The specific surface it provides is so large that more CO 2 molecules have a chance to contact the catalyst for reduction reactions.
Growing attention has been paid to hierarchical composite pores, including photonic crystal catalysis and separation of sub-microns. The slow light effect of photons associated with 3DOM materials have been considered to increase solar radiation absorption and enhance photocatalyst performance [21]. 3DOM products with periodic macrostructures [22], known as inverse opal, have been applied in battery materials, sensors, separation engineering and heterogeneous catalysis. Thanks to the periodic dielectric constants, Bragg diffraction permits certain wavelengths of light to radiate, leading to stop-band reflection. The limited photons reflected back will slow down at the edge of the stop bands, from which it received its name of "slow photons" [23,24]. It accelerates the light absorption rate when the photon energy is consistent with the absorption spectrum of the 3DOM materials [25]. In addition, the light scattering properties are optimized due to the relation between the macrospores and pore walls. For example, a 3D macro mesoporous Mo: BiVO 4 architecture [26] was designed and fabricated, which had a superior photocurrent density.
It has been demonstrated that gas absorption and separation functions benefit from orderly porous architectures [27]. 3DOM chemicals show superior CO 2 absorption/desorption rates than other commercial products, as the special structure can capture CO 2 from the ambient air by utilizing humidity variation. It is a great way to use microporous catalysts because it not only increases visible light absorption but also shortens the transport time of CO 2 . They are available at a low cost through simple and rapid methods.

Preparation of 3DOM Materials
Specifically, 3DOM materials are produced by the colloidal crystal template (CCT) method ( Figure 4). A uniform and close-connected organic sphere template can be obtained, firstly, through three key processes. Then, seep metallic salt sol into the void of microspheres. After heat treatment, the organic microsphere template fades away and leaves a metal oxide frame. (i) The polymerizable monomer (methyl methacrylate, styrene) and initiator are mixed and heated under the protection of Ar; (ii) the earlier reaction liquid is filtered with microfiltration membrane; (iii) the microsphere mixture is centrifuged at a high speed for a long time, yielding the polymethylmethacrylate (PMMA) or polystyrene (PS) template; (iv) the template is immersed in the precursor solution and (v) is calcinated. enhance photocatalyst performance [21]. 3DOM products w [22], known as inverse opal, have been applied in battery m engineering and heterogeneous catalysis. Thanks to the p Bragg diffraction permits certain wavelengths of light to rad flection. The limited photons reflected back will slow down a from which it received its name of "slow photons" [23,24]. It tion rate when the photon energy is consistent with the absor materials [25]. In addition, the light scattering properties are o between the macrospores and pore walls. For example, a BiVO4 architecture [26] was designed and fabricated, which density.
It has been demonstrated that gas absorption and separ orderly porous architectures [27]. 3DOM chemicals show s sorption rates than other commercial products, as the speci from the ambient air by utilizing humidity variation. It is a g catalysts because it not only increases visible light absor transport time of CO2. They are available at a low cost throug 2.1.5. Preparation of 3DOM Materials Specifically, 3DOM materials are produced by the collo method ( Figure 4). A uniform and close-connected organic tained, firstly, through three key processes. Then, seep met microspheres. After heat treatment, the organic microsphe leaves a metal oxide frame. (i) The polymerizable monome rene) and initiator are mixed and heated under the protection liquid is filtered with microfiltration membrane; (iii) the m fuged at a high speed for a long time, yielding the polymet polystyrene (PS) template; (iv) the template is immersed in th is calcinated. In the absence of ultra-high temperature and expensive e structure was obtained. Various materials of frame can also b precursor formula.  In the absence of ultra-high temperature and expensive equipment, a fantastic 3DOM structure was obtained. Various materials of frame can also be synthesized by altering the precursor formula.

Heterojunction
Photogenerated charge carriers in a single material tend to recombine rapidly due to coulombic force and they can be separated efficiently in the multi-materials that are in close contact [28]. The heterojunction is an interface between various semiconductors with different energy band structures. The heterojunction structure can reduce the probability of charge recombination and is therefore considered an effective method to enhance the photocatalytic activity [29,30].

Defects
All semiconductors have surface defects, which are rooted in the absence of host atoms. The amount of oxygen vacancies can be altered by ion doping and nanocrystal modifications. Oxygen defects and other defects originating from the generated vacancy [31] can facilitate the separation of h + /e − . Over and above, defects can also activate CO 2 molecules, reducing the activation energy of the reaction [32].

Ion Doping
Elemental doping is a common strategy to modulate the surface electronic structure. Then, the band gap of the semiconductors make a difference, nonmetal doping (such as N, C and O) mainly alters the VB and metal-doping (e.g., Mo, Co and Ni) influences the CB [33,34]. For example, Cu-doped TiO 2 absorbs more visible light due to the Cu 3d-Ti 3d optical transition [35].

Sensitization
Sensitization means coupling the quantum dots, dyes, etc., with semiconductors to increase the photogenerated carriers and promote the absorption of light by taking advantage of their receptivity in UV, visible or infrared light.

TiO 2 -Based Photocatalysts
Titanium dioxide (TiO 2 ) is notable in photochemistry, with advantages such as nontoxic, cheapness, corrosion resistant, good physical and chemical stability. However, owing to the wide Eg of 3.0-3.2 eV, TiO 2 only absorbs energy in the ultraviolet region (3-5% of the solar energy), and photoexcited charge pairs are easy to combine, resulting in quantum inefficiency. Usually, TiO 2 is divided into the rutile phase and anatase phase on the basis of atomic arrangement modes. Rutile TiO 2 is thermodynamically stable and does not distort or decompose at high temperatures. It has a narrower energy gap (3.0 eV) and a wider spectral response than the anatase phase (3.2 eV). Rutile TiO 2 seeds generally grow larger in size and tend to form an agglomerated structure. Smaller anatase TiO 2 particles have a wider lattice gap and abundant surface oxygen defects, which make it favorable for ion doping and photoreactions.

Morphology Control
In hollow nanotube-shaped catalysts, the transport speed of CO 2 and photoproducts can be facilitated. Ru phase is inclined to form methane in CO 2 hydrogenation process [36] and Yang et al. [37] entrapped Ru nanoparticles in TiO 2 nanotubes. Restricted Ru nanoparticles were resistant to sintering and leaching in the Ru-in/TNT catalyst channel ( Figure 5). Electrons tend to gather in the tubes because of a confinement effect, which leads to an abundant, accessible metallic phase. It is easier for high-priced Ru species to combine with free electrons and then exhibit a superb CH 4 and CH 3 OH yield.
olecules 2023, 28, x FOR PEER REVIEW Figure 5. The reaction pathway for the Ru entrapped in the TiO2 nanotubes catalyze tion [37].
On the contrary, Kar et al. [38] loaded metal nanoparticles onto vert 1D TiO2 nanotube arrays (TNAs) platforms via the graft method. In the syn Au, Ru and ZnPd NPs grow anodically on transparent glass substrates. Th bending phenomenon in Au NP-grafted TiO2, which can be observed fr photoelectron spectroscopy (UPS). TPD experiments proved that all NP-gr absorb more CO2 than TNAs. It is worth noting that, in nanoparticle-graft photons close to and below the TiO2 band edge were excited to drive CO2 p process. Ru-TNAs, ZnPd-TNAs and Au-TNAs had the CH4 formation rate 58 µmol·g -1 ·h -1 , respectively. Pt and CoOx growing on the outer and inner rous TiO2-SiO2 frame, respectively, can act as "warehouses" for e − and h + , tivity of the new system for reducing CO2 to CH4 can reach 94% [39].
Metal-organic frameworks (MOFs), also known as porous coordinat composed of organic linkers and metal nodes (metal ions or clusters), are a crystalline inorganic-organic hybrid materials. The unique advantages of extremely high surface area, uniform adjustable porous structure and high dination unsaturated metal sites, have been extensively researched. They plied in many fields, such as gas adsorption and separation, sensing, cat and conversion of CO2. The high specific surface area and uniform porou MOFs make it possible to incorporate metal nanoparticles (as electron accep frameworks, leading to efficient charge separation. In addition, the eminen tion capacity of various MOFs resulted in higher concentration of CO2 in th helped to accelerate the photoreaction. In [40], a porous material-zirconium skeleton (UiO-66) was introduced to a TiO2 photocatalyst as an effective C The designed two-step strategy endowed the TiO2/UiO-66 composite w graded pore structure, thus ensuring sufficient catalytic sites and high C capacity (78.9 cm 3 g −1 ). The ultrafine TiO2 nanoparticles were loosely loade 66 surface rather than tightly packed due to the electrostatic repulsion, thu exist of microporous of the MOF. Finally, in the weak gas-solid catalyti −1 −1 Figure 5. The reaction pathway for the Ru entrapped in the TiO 2 nanotubes catalyzes CO 2 methanation [37].
On the contrary, Kar et al. [38] loaded metal nanoparticles onto vertically oriented 1D TiO 2 nanotube arrays (TNAs) platforms via the graft method. In the synthesis process, Au, Ru and ZnPd NPs grow anodically on transparent glass substrates. There is no band bending phenomenon in Au NP-grafted TiO 2 , which can be observed from ultraviolet photoelectron spectroscopy (UPS). TPD experiments proved that all NP-grafted samples absorb more CO 2 than TNAs. It is worth noting that, in nanoparticle-grafted TNAs, blue photons close to and below the TiO 2 band edge were excited to drive CO 2 photoreduction process. Ru-TNAs, ZnPd-TNAs and Au-TNAs had the CH 4 formation rates of 26, 27 and 58 µmol·g -1 ·h -1 , respectively. Pt and CoO x growing on the outer and inner layers of a porous TiO 2 -SiO 2 frame, respectively, can act as "warehouses" for e − and h + , and the selectivity of the new system for reducing CO 2 to CH 4 can reach 94% [39].
Metal-organic frameworks (MOFs), also known as porous coordination polymers, composed of organic linkers and metal nodes (metal ions or clusters), are a kind of porous crystalline inorganic-organic hybrid materials. The unique advantages of MOFs, such as extremely high surface area, uniform adjustable porous structure and high density coordination unsaturated metal sites, have been extensively researched. They have been applied in many fields, such as gas adsorption and separation, sensing, catalysis, capture and conversion of CO 2 . The high specific surface area and uniform porous structure of MOFs make it possible to incorporate metal nanoparticles (as electron acceptors) into their frameworks, leading to efficient charge separation. In addition, the eminent CO 2 adsorption capacity of various MOFs resulted in higher concentration of CO 2 in the pores, which helped to accelerate the photoreaction. In [40], a porous material-zirconium-based organic skeleton (UiO-66) was introduced to a TiO 2 photocatalyst as an effective CO 2 adsorbent. The designed two-step strategy endowed the TiO 2 /UiO-66 composite with abundant graded pore structure, thus ensuring sufficient catalytic sites and high CO 2 adsorption capacity (78.9 cm 3 g −1 ). The ultrafine TiO 2 nanoparticles were loosely loaded on the UiO-66 surface rather than tightly packed due to the electrostatic repulsion, thus ensuring the exist of microporous of the MOF. Finally, in the weak gas-solid catalytic system with water as the electron donor, the yield of CH 4 was up to 17.9 µmol g −1 h −1 , and the selectivity was 90.4%. Additionally, the photocatalytic efficiency was comparable to that of pure CO 2 atmosphere even under low CO 2 concentration conditions (≤2%).

Exposed Facet Adjustment
A Pt-TiO 2 single atomic site catalyst (PtSA/Def-s-TiO 2 ) was prepared [41] by the "thermal solvent-argon treatment and hydrogen reduction" method. In order to construct Ti-Pt-Ti structures, TiO 2 nanosheets with oxygen deficient sites were used to anchor monatomic Pt particles, which can retain the stability of isolated single atomic Pt and improve photocatalytic performance. The exposed (101) and (001) crystalline of TiO 2 nanosheets ( Figure 6A,B) were determined by transmission electron microscopy, and a thickness of 6.9 nm was observed through atomic force microscope. The EPR spectra of the samples confirm that the rich oxygen defect structure can be obtained by heating TiO 2 nanosheets in argon atmosphere. They also indicated that single-atom Pt junctions were formed by occupying the oxygen defect sites, which provides a benchmark for the rational design of highly active and stable single-atom catalysts on metal oxide carriers with defect structures. CH 4 product can be detected when copper oxide nanoparticles are mixed with mesoporous TiO 2 nanorods in close contact [42]. Both the special crystal plane and porous structure contribute to furthering the CH 4 yield.
les 2023, 28, x FOR PEER REVIEW confirm that the rich oxygen defect structure can be obtained by h in argon atmosphere. They also indicated that single-atom Pt jun occupying the oxygen defect sites, which provides a benchmark f highly active and stable single-atom catalysts on metal oxide ca tures. CH4 product can be detected when copper oxide nanopartic oporous TiO2 nanorods in close contact [42]. Both the special cr structure contribute to furthering the CH4 yield.

3DOM Structure Ti-Based Materials
A ternary 3DOM Bi-doped TiO2 photocatalyst decorated with obtained, whose pore engineering of the 3DOM skeleton greatly in the whole solar spectrum range [26]. It exhibits enhanced pho because of its excellent exquisite structure and high charge trans a BiVO4/3DOM TiO2 nanocomposite [43] was synthesized as a hig lytic catalyst for the degradation of dye pollutants. Further studie and surface properties revealed that connecting pores not only imp fer rate between coupled materials, but also provide abundant molecules.
3DOM TiO2 were prepared [44] by the CCT method (Figure 7 samples ( Figure 7B) were obtained by the original bubble-assiste tion method. The introduction of CeO2 nanolayers broadened the and facilitated the separation of photogenerated electron−hole structure provided a larger surface area and the catalyst exhibited activity.

3DOM Structure Ti-Based Materials
A ternary 3DOM Bi-doped TiO 2 photocatalyst decorated with carbon dots (CDs) was obtained, whose pore engineering of the 3DOM skeleton greatly promoted the response in the whole solar spectrum range [26]. It exhibits enhanced photocatalytic performance because of its excellent exquisite structure and high charge transfer efficiency. Similarly, a BiVO 4 /3DOM TiO 2 nanocomposite [43] was synthesized as a highly efficient photocatalytic catalyst for the degradation of dye pollutants. Further studies on its textural, optical and surface properties revealed that connecting pores not only improve the electron transfer rate between coupled materials, but also provide abundant active sites for reactant molecules.
3DOM TiO 2 were prepared [44] by the CCT method ( Figure 7A) and CeO 2 /3DOM TiO 2 samples ( Figure 7B) were obtained by the original bubble-assisted membrane precipitation method. The introduction of CeO 2 nanolayers broadened the photo-absorption range and facilitated the separation of photogenerated electron−hole pairs. The mesoporous structure provided a larger surface area and the catalyst exhibited a higher CO 2 reduction activity.
confirm that the rich oxygen defect structure can be obtained by hea in argon atmosphere. They also indicated that single-atom Pt junct occupying the oxygen defect sites, which provides a benchmark for highly active and stable single-atom catalysts on metal oxide carri tures. CH4 product can be detected when copper oxide nanoparticle oporous TiO2 nanorods in close contact [42]. Both the special crys structure contribute to furthering the CH4 yield.

3DOM Structure Ti-Based Materials
A ternary 3DOM Bi-doped TiO2 photocatalyst decorated with c obtained, whose pore engineering of the 3DOM skeleton greatly pr in the whole solar spectrum range [26]. It exhibits enhanced photo because of its excellent exquisite structure and high charge transfer a BiVO4/3DOM TiO2 nanocomposite [43] was synthesized as a high lytic catalyst for the degradation of dye pollutants. Further studies and surface properties revealed that connecting pores not only impro fer rate between coupled materials, but also provide abundant ac molecules.
3DOM TiO2 were prepared [44] by the CCT method (Figure 7A samples ( Figure 7B) were obtained by the original bubble-assisted tion method. The introduction of CeO2 nanolayers broadened the ph and facilitated the separation of photogenerated electron−hole pa structure provided a larger surface area and the catalyst exhibited a activity.  Pt-particle-decorated 3DOM carbon-coated TiO 2 and g-C 3 N 4 were combined [45] to construct an all-solid-state catalyst for CO 2 artificial photoconversion. Slow photon effect and carbon coat optimized the absorption capability of light. The exquisite design drove vectorial electrons from TiO 2 @C to Pt particles and then fell to g-C 3 N 4 , which facilitated the carrier separation. The Z-scheme that consist of two isolated systems has three components and converts CO 2 to CH 4 with H 2 O at a yield of 65.6 µmol g −1 h −1 . To enhance the surface enrichment of CO 2 , Wu et al. [46] fabricated 3DOM perovskite-type Pt n /SrTiO 3 , in which Pt nanoparticles can take in photoelectrons from SrTiO 3 and transfer CO 2 to CO and CH 4 ( Figure 8). Pt 2 /3DOM SrTiO 3 exhibited the highest CH 4 yield of 26.7 µmol g −1 h −1 .
es 2023, 28, x FOR PEER REVIEW Figure 8. The possible mechanisms of the photocatalytic CO2 reduction A two-dimensional MoS2 layers/3DOM TiO2 photocatalyst heterojunctions, which had a higher performance in the range atoms were uniformly distributed in 3DOM TiO2 via the in situ vides active sites for CO2 photoconversion. The main products te phase system were CH4 and C2H4 with the corresponding gen 6.99 µmol g −1 h −1 . Chen's work provided a new perspective fo efficiency by regulating reaction conditions. Jiao et al. [49] lo AuPd NPs in 3DOM TiO2 via one pot of the gas bubbling-assi method to form a functional photocatalyst. The low Fermi level the catalyst system to trap electrons and enhance the separation quantum-dot-decorated 3DOM CaTiO3 photocatalysts were dul an apparent quantum efficiency (QAY). The macro-meso-mic vided improved charge carrier separation and transport, and through density functional theory calculations (DFT) and finit simulations. A two-dimensional MoS 2 layers/3DOM TiO 2 photocatalyst was prepared [47] to form heterojunctions, which had a higher performance in the range of 420-900 nm. Cu single atoms were uniformly distributed in 3DOM TiO 2 via the in situ method [48], which provides active sites for CO 2 photoconversion. The main products tested in the gas-solid two-phase system were CH 4 and C 2 H 4 with the corresponding generation rates of 43.15 and 6.99 µmol g −1 h −1 . Chen's work provided a new perspective for improving the catalytic efficiency by regulating reaction conditions. Jiao et al. [49] loaded core-shell structure AuPd NPs in 3DOM TiO 2 via one pot of the gas bubbling-assisted membrane reduction method to form a functional photocatalyst. The low Fermi level of AuPd NPs empowered the catalyst system to trap electrons and enhance the separation of charge pairs. Carbon-quantum-dotdecorated 3DOM CaTiO 3 photocatalysts were duly obtained [50], exhibiting an apparent quantum efficiency (QAY). The macro-meso-microporosity structure provided improved charge carrier separation and transport, and it was explored in depth through density functional theory calculations (DFT) and finite difference time-domain simulations.

Heterojunction
p-n heterojunction: A p-n heterojunction is formed by combining p-type and n-type semiconductors. Even without light irradiation, electrons can diffuse from an n-type semiconductor to a nearby p-type one, in the case of the combination of two materials. Correspondingly, the holes on the surface of a p-type semiconductor are transferred to the n-type one, which results in an efficient separation of charge carriers. A ZnFe 2 O 4 -modified TiO 2 was synthesized by the hydrothermal method [51], and the p-n heterojunction system could reduce CO 2 to methanol at a yield of 75.34 µmol g −1 h −1 .
rGO composite: In recent years, graphene materials have been widely used because of their large specific surface area, unique thermal stability and excellent electrical conductivity. Graphite nanomaterials are visible-light-responsive materials with appropriate band gaps, and the energy levels of CB and VB are in optimal positions relative to ordinary hydrogen electrodes. These unique photocatalytic properties have made them prime candidates for photocatalytic CO 2 reduction. Fortunately, tightly contacted ultra-thin graphene layers and TiO 2 compounds and can be prepared with some additives [52]. Seeharaj et al. [53] employed high-intensity ultrasonic waves (ultrasonic horn, 20 kHz, 150 W/cm 2 ) to exfoliate the TiO 2 surface, which led to a highly specific active area and highly reactive nanosheets. The modification of tiny rGO and CeO 2 on the rGO nanosheet surface can improve the CO 2 absorptivity and the charge carriers' migration efficiency of the catalyst (Figure 9). A kind of d-π electron orbital overlap was formed between TiO 2 s d orbitals and rGO's π orbitals, which provides a good environment for activated CO 2 and electrons. The complex heterojunction photocatalysts TiO 2 /rGO/CeO 2 exhibited high yields of CH 3 OH (641 µmol/gcath) and C 2 H 5 OH (271 µmol/gcath).
BCN composite: The boron carbon nitride (BCN) composite BC band gap material that has been applied in CO2 reduction, water splitti ification catalyst. Highly negative CB potential of BCN materials make the construction of S-scheme heterojunction, and Kumar et al. [54] d scheme Fe@TiO2/BCN composite. In situ XPS technology, DFT calculat ference time-domain simulations were adopted to verify the S-schem nism. Under visible light irradiation, the intimately contacted heteroju probability of charge recombination. The sample exhibited excellent ph ity; in addition to converting CO2 selectively, it also degraded tetracycl TMS composite: Some transition metal sulfides (TMSs) exhibit th as Pt or Pd in photocatalyst. They form longer-lived charged carriers w [55] with a highly negative reduction potential. However, the self-oxid fides confuses researchers and limits their application. It can be alleviat a Z-scheme, and thus researchers introduced it to Bi-modified TiO2 [ spatially coupled heterojunction was enhanced by regularly capsulat BCN composite: The boron carbon nitride (BCN) composite BCN is an adjustable band gap material that has been applied in CO 2 reduction, water splitting and as a detoxification catalyst. Highly negative CB potential of BCN materials makes them suitable for the construction of S-scheme heterojunction, and Kumar et al. [54] designed a novel S-scheme Fe@TiO 2 /BCN composite. In situ XPS technology, DFT calculations and finite difference time-domain simulations were adopted to verify the S-scheme transfer mechanism. Under visible light irradiation, the intimately contacted heterojunction reduced the probability of charge recombination. The sample exhibited excellent photocatalytic activity; in addition to converting CO 2 selectively, it also degraded tetracycline antibiotics.
TMS composite: Some transition metal sulfides (TMSs) exhibit the same properties as Pt or Pd in photocatalyst. They form longer-lived charged carriers within the S 3p orbital [55] with a highly negative reduction potential. However, the self-oxidation of metal sulfides confuses researchers and limits their application. It can be alleviated by constructing a Z-scheme, and thus researchers introduced it to Bi-modified TiO 2 [56]. Meanwhile, a spatially coupled heterojunction was enhanced by regularly capsulated CuCo 2 S 4 yolk-shell hollow sphere.

Ion Doping
In recent years, elements such as B, N, Co and Bi have been widely applied in TiO 2 doping. A carbon-based hybrid nanocomposite reduced graphene oxide (rGO), belonging to the narrow band gap, with oxygen-containing functional groups on the surface that can be enhanced by π interactions [57]. Laminar graphene carriers not only prevent TiO 2 repolymerization, but also hybridize the function of the catalytic system. Co-doped TiO 2 was loaded on the rGO [58], and the Co peak in EDX spectra and C-O peak in FT-IR spectra confirmed the successful doping and the presence of graphene support, respectively. The size of TiO 2 particles decreased from 48-80 nm to 23-28 nm, which is consistent with earlier reports of changes in titanium doping with transition metal ions.

Sensitization
A growing number of semiconductor materials are being used to modify TiO 2 dioxide, but randomly mixed catalysts are not stable enough to achieve reproducibility. Therefore, Lee [59] grew well dispersed p-type NiS nanoparticles on the surface of a highly aligned n-type TiO 2 film to obtain the NiS-sensitized TiO 2 films. The band gaps of two components were estimated by wavelength relation. Some inferences can be drawn when considering the results of both the ultraviolet and visible spectra. It indicates that more electrons are subpoenaed from the short-Eg NiS and transferred to TiO 2 conduction band. The spectra results reconfirmed the electron contribution of the NiS and the design of a catalyst that produced 3.77-fold CH 4 compared to the TiO 2 film.

Summary
Overall, some of the photocatalytic systems that use TiO 2 is presented in Table 1.

WO 3 -Based Photocatalysts
Tungsten-based oxides (WO 3 ) have been extensively studied in recent decades and various morphologies have been presented. In the WO 3 structure, the crystal in the stoichiometric ratio is connected with a twisted WO 6 octahedra to form a perovskite crystal structure. It has monoclinic, orthorhombic and hexagonal crystal forms. At the same time, the oxygen lattice can be lost easily, resulting in oxygen vacancies and unsaturated, coordinated W atoms. Therefore, tungsten oxide has many non-stoichiometric compounds, such as WO 2.72 , WO 2.8 , WO 2.83 and WO 2.9 . Of these, WO 3 is the most common and has been widely studied as a typical photocatalytic water oxidation semiconductor material. WO 3 is a typical narrow-band gap indirect semiconductor with a forbidden band width of 2.6-2.8 eV, which can absorb part of the visible light [64]. In addition, WO 3 is a research hotspot in the field of photoelectrochemical water splitting because of its high carrier mobility, stability in acidic electrolytes and resistance to photocorrosion.

Morphology Control
Bi 2 WO 6 is one of the tungsten-based materials that belongs to Aurivillius crystal oxides. Its crystal has an orthorhombic system, and its narrow band gap (2.7-2.9 eV) structure allows it to meet the response absorption of visible light. Moreover, its stable structure and eco-friendly properties have attracted many scientific researchers to study it. Since the valence band of Bi 2 WO 6 is composed of O 2p and Bi 6p , and the conduction band is composed of W 5d -assisted Bi6p orbitals, the VB energy levels can be dispersed broadly. By employing the Kirkendall effect in ion exchange and BiOBr precursor, Huang et al. [65] prepared a bowl-shaped Bi 2 WO 6 HMS material. Based on the large specific surface area of the material, its adsorption capacity for CO 2 reaches 12.7 mg g −1 at room temperature and pressure. The material adsorbs a large number of HCO 3 − and CO 3 2− species on the surface during the reaction, which makes the catalytic reaction easier. The Bi 2 WO 6 HMS thus has a high catalytic activity, and the methanol yield is 25 times higher than that of the Bi 2 WO 6 SSR.
Iron phthalocyanine FePc is neatly assembled on porous WO 3 under induction and coupled with surface atoms by H-bonding [66]. The optimized FePc/porous WO 3 nanocomposites exhibit enhanced CO 2 photoreduction activity, which is attributed to the synergistic effects of a high specific surface area, a better charge separation and proper central metal cation. A series of mesoporous WO 3 with interconnected networks were synthesized by the silica KIT-6 hard template method [67], which became oxygen-deficient after hydrogenation treatment. Both the ordered porous structure and oxygen vacancies contributed to the increased yield of CH 4 and CH 3 OH. WO 3 with a hollow nest morphology with hierarchical micro/nanostructures (HN-WMs) was synthesized [68] by the one-step hydrothermal method (Figure 10), with a particle diameter of about 2.5 µm. The 2D nanosheets, which have an average thickness of 30-40 nm, were assembled to build a distinctive hollow nest structure with a good stability and reusability under visible light. Hao et al. [69] prepared core-shell heterojunctions of two-dimensional lamellar WO 3 /CuWO 4 by the in situ method. After the modification of amorphous Co-Pi co-catalyst, the photoanode of ternary homogeneous core-shell structure exhibited a high photocurrent of 1.4 mA/cm 2 at 1.23 V/RHE, which was 6.67 and 1.75 times higher than that of the pristine WO 3 and 2D homogeneous heterojunction. Ren et al. [70] synthesized unique flower-like Bi 2 WO 6 /BiOBr catalysts by the simple onestep solvothermal method, and showed that the photocatalytic activity of the composites was significantly enhanced due to the construction of type II heterojunctions. The presence of Br source enhanced the light absorption and improved charge-carrying spatial transfer and separation. and 1.75 times higher than that of the pristine WO3 and 2D homogeneous heterojunction. Ren et al. [70] synthesized unique flower-like Bi2WO6/BiOBr catalysts by the simple onestep solvothermal method, and showed that the photocatalytic activity of the composites was significantly enhanced due to the construction of type II heterojunctions. The presence of Br source enhanced the light absorption and improved charge-carrying spatial transfer and separation. Ti atoms in ultrathin Ti-doped WO3 nanosheets promoted the charge transfer [71], as they accelerate the generation of key intermediates COOH*, which was revealed by in situ characterization. Furthermore, Gibbs free energy calculations were calculated to verify that ion doping can reduce the CO2 activation energy barrier and CH3OH desorption energy barrier by 0.22 eV and 0.42 eV, respectively, thus promoting the formation of CH3OH. The ultrathin Ti-WO3 nanosheets showed an excellent CH3OH yield of 16.8 µmol g −1 h −1 . Two-dimensional bilayered WO3@CoWO4 were prepared [72] via a facile interface-induced synthesis method. The optical energy conversion efficiency can be improved by both p-n heterojunctions and interfacial oxygen vacancies. The narrow band gap of the Ti atoms in ultrathin Ti-doped WO 3 nanosheets promoted the charge transfer [71], as they accelerate the generation of key intermediates COOH*, which was revealed by in situ characterization. Furthermore, Gibbs free energy calculations were calculated to verify that ion doping can reduce the CO 2 activation energy barrier and CH 3 OH desorption energy barrier by 0.22 eV and 0.42 eV, respectively, thus promoting the formation of CH 3 OH. The ultrathin Ti-WO 3 nanosheets showed an excellent CH 3 OH yield of 16.8 µmol g −1 h −1 . Two-dimensional bilayered WO 3 @CoWO 4 were prepared [72] via a facile interface-induced synthesis method. The optical energy conversion efficiency can be improved by both p-n heterojunctions and interfacial oxygen vacancies. The narrow band gap of the WO 3 @CoWO 4 heterojunction was proved by DFT calculations and some characterizations, which allows a better visible light absorption. A tree-like WO 3 film was prepared [73] by the hydrothermal process, which has a large specific surface area. The WO 3 product was a unity of hexagonal/monoclinic crystals, which contained W 5+ defects and oxygen vacancies. The products were further subjected to a mild reduction solution at the lower temperature of 333 K to introduce more defects. It turns out that the intermediate state induced by defects diminished the band gap. A reasonable amount of defects benefits the photocatalytic activity of WO 3 , while too many defects impair its catalytic capacity. The performance of the treated WO 3 films increased 2.1 times in 48 h compared to that of the annealed WO 3 samples.

Preferentially Exposed Facets
According to studies, infrared (IR) light makes up nearly 50% of solar energy, and it is challenging to make use of the majority of the light. Liang et al. [74] fabricated 2D ultrathin WO 3 with an intermediate band gap. They achieved the first complete decomposition of CO 2 driven by infrared light without the addition of sacrificial agents. Theoretical calculations indicated that the generation of the intermediate energy band resulted from the critical density of the generated oxygen vacancies, which has also been verified by synchrotron valence band spectroscopy, photoluminescence spectroscopy, ultraviolet-visible-near-infrared spectroscopy and synchrotron infrared reflection spectroscopy. The results showed that the WO 3 atomic layer containing oxygen vacancies can achieve the complete decomposition of CO 2 and generate CO and O 2 under infrared light.
Microscopic WO 3 nanocrystals were formed by Chen et al. [75] through solid-liquid phase arc discharge in an aqueous solution. Then, they synthesized ultrathin single-crystal WO 3 nanosheets via a laterally oriented attachment method. The quantization effect of this nanostructure altered the bandgap width of WO 3 nanosheets, enabling the semiconductor to exhibit a high performance at a wide range of nanometer sizes. It is beneficial to control the activity and selectivity of the photoconverted CO 2 products. As a consequence, WO 3 with a strong visible light response has enormous potential in the field of photocatalytic CO 2 reduction. Bi 2 WO 6 has a positive conduction band potential, which is not enough to excite and reduce CO 2 molecules, thus limiting its application in the photocatalytic reduction of CO 2 . Therefore, researchers have been devoted to modifying the surface of Bi 2 WO 6 to improve its photocatalytic activity in order to obtain a high-efficiency CO 2 reduction ability. Zhou et al. [76] successfully prepared Bi 2 WO 6 nanosheets with a monolayered structure by introducing the surfactant CTAB (hexadecyltrimethylammonium bromide) into the precursor solution. A large number of unsaturated Bi atoms were produced, which provided sufficient active sites for photocatalytic reactions. Bi 2 WO 6 is composed of a With the assistance of an oil-based primary amine (C 18 H 37 N) surfactant, Zhou et al. [77] conducted a hydrothermal reaction at 200 • C for 20 h to prepare an ultra-thin and uniform Bi 2 WO 6 nanosheet. The material has a strong response under visible light, and its forbidden band width was about 2.44 eV through theoretical calculations, with a conduction band potential of −0.31 e V. Then, CO 2 could be easily reduced to CH 4 , and its yield was 20 times higher than that of SSR Bi 2 WO 6 . The light-absorbing capacity will decrease if the nanosheets are too thin because of the quantum size effect. Therefore, scholars need to consider roundly when designing fresh material [78].

DOM Structure W-Based Materials
Unexpectedly, it was found that the resistance of 3DOM-WO 3 (270) and the Ag 3 PO 4 electron absorption band were comparable. By depositing Ag 3 PO 4 nanoparticles in the micropores of 3DOM-WO 3 , Chang et al. [79] achieved a higher photocatalytic activity and more efficient light harvesting at the wavelengths of 460-550 nm. A Z-scheme g-C 3 N 4 /3DOM-WO 3 catalyst designed by Tang et al. [80] also has a high CO 2 photoreduction activity. The separation way of the photogenerated electron-hole pairs determined its Z structure, and 3DOM framework heightened the light collecting efficiency. Therefore, an excellent photocatalyst exhibited a high CO evolution rate of 48.7 µmol g −1 h −1 .

Heterojunction
Quantum dot composite: CuO quantum dots (QDs) were combined with WO 3 nanosheets by a self-assembly method and the diameter of 6%CuO QDs/WO 3 NSs was mainly located at 1.6 nm [81]. The bandgap energy of CuO/WO 3 fell in 2.28 eV and the complex catalyst possessed a lower resistance for charge carrier transfer that showed in UV-vis DRS and EIS analysis. Due to the low CB position, CO cannot be obtained when using pure WO 3 . However, the photogenerated electrons gathered in the WO 3 CB position was able to reach the CuO VB position when the Z-scheme ( Figure 11) was formed by intimate heterojunctions. At the same time, the reduction reaction that transformed CO 2 into CO occurred at the CuO CB position. The high yield rate of about 1.58 mmol g −1 h −1 also benefited from a longer fluorescence lifetime, and reduced the overlap of electron pore pairs. lecules 2023, 28, x FOR PEER REVIEW DOM Structure W-Based Materials Unexpectedly, it was found that the resistance of 3DOM-WO3(270) and electron absorption band were comparable. By depositing Ag3PO4 nanopar micropores of 3DOM-WO3, Chang et al. [79] achieved a higher photocatalytic more efficient light harvesting at the wavelengths of 460-550 nm. A Z C3N4/3DOM-WO3 catalyst designed by Tang et al. [80] also has a high CO2 pho activity. The separation way of the photogenerated electron-hole pairs deter structure, and 3DOM framework heightened the light collecting efficiency. T excellent photocatalyst exhibited a high CO evolution rate of 48.7 µmol g −1 h

Heterojunction
Quantum dot composite: CuO quantum dots (QDs) were combined nanosheets by a self-assembly method and the diameter of 6%CuO QDs/W mainly located at 1.6 nm [81]. The bandgap energy of CuO/WO3 fell in 2.28 complex catalyst possessed a lower resistance for charge carrier transfer tha UV-vis DRS and EIS analysis. Due to the low CB position, CO cannot be obt using pure WO3. However, the photogenerated electrons gathered in the WO3 was able to reach the CuO VB position when the Z-scheme ( Figure 11) wa intimate heterojunctions. At the same time, the reduction reaction that trans into CO occurred at the CuO CB position. The high yield rate of about 1.58 also benefited from a longer fluorescence lifetime, and reduced the overlap pore pairs. Perovskite composite: For WO3, the negative potential energy of the (−31.5 mV) was lower than that of the rod (−21.0 mV). In addition to its p Perovskite composite: For WO 3 , the negative potential energy of the sheet shape (−31.5 mV) was lower than that of the rod (−21.0 mV). In addition to its potential advantages, S-WO 3 offers a higher specific surface area. Defects occurred because of the exposed interior atoms in nanosheets surface, which promoted the CO 2 adsorption. Positively charged perovskite cesium lead tribromide (CsPbBr 3 , CPB) with a long electrically diffused length was selected to be combined with S-WO 3 , affording a high field rate of CO and CH 4. Zhang and others [82] employed a three-dimensional hydrophobic porous melamine foam to support the mixture, not only to protect the CPB from dissolution but also to reduce toxic Pb 2+ .

Ion Doping
Molybdenum with a similar ionic size was chosen to dope the WO 3 as a low-valence metal species. The W 5+ /W 6+ ratio of the catalyst was increased, making it easier to exchange electrons with reactants. The conductivity of protons was enhanced by the presence of hydrogen bronze, which originated from a chemical reaction between WO 3 and Brønsted protons and excess electrons in their lattices. Wang et al. [83] prepared molybdenumdoped WO 3 ·0.33 H 2 O by the hydrothermal method. The E cb and E vb energy were both higher at 3%Mo-WO than WO 3 . After 20 min of the FTIR spectra, rare intermediate CO 2− was observed, verifying the activation of CO 2 , which was more common in lowvalent meta species. The content of potassium hydroxide in an aqueous solution obtained by photocatalytic water oxidation was higher and the CH 4 yield was 4.2 times higher than WO 3 .

Summary
To date, Bi 2 WO 6 semiconductor photocatalytic materials have made great progress in the field of environmental management. Some photocatalytic systems using WO 3 -based materials in CO 2 conversion are listed in Table 2.

ZnO-Based Photocatalysts
ZnO, a common metal oxide, is a n-type semiconductor with an Eg value of 3.37 eV. It is a kind of amphoteric oxide that has the advantages of nontoxic harmlessness, low cost, abundant reserves, convenient preparation, low dielectric constant and low optical coupling rate. ZnO has three main lattice structures: wurtzite structure, zinc-blended structure and tetragonal rock salt structure. The wurtzite structure is considered the most stable and common structure in nature. It is a kind of hexagonal crystal, in which the O and Zn atoms are aligned with the hexagonal density stacking. The photodegradation process of ZnO is similar to TiO 2 and it has been widely used in photocatalysts, solar cells and conducting materials.

Morphology Control
The 3nm Pt particles were uniformly dispersed over ZnS@ZnO with a mesoporous heterostructure [90] and more CH 3 OH was obtained. Reactant charge carriers entered the pore channels of the porous heterozygous layer, thus reducing the likelihood of flow resistance and electron-hole recombination. The S-scheme photocatalyst delivered a high CH 3 OH formation rate of 81.1 µmol g −1 h −1 , which is roughly 40 and 20 times larger than that of bare ZnO (3.72 µmol g −1 h −1 ) and ZnO-ZnS (4.15 µmol g −1 h −1 ). On the other hand, a porous ZnO@ZnSe core/shell nanosheet array material ( Figure 12A) was prepared in a controlled manner [91]. The final n-type semiconductor composites had a proper negative CB band edge. In comparison to ZnO or ZnSe, more pairs of electronholes were formed under visible light. Electrons tend to land on ZnO, which is aimed at methanol production. Mei et al. prepared a ZnO microsphere with different numbers of shells [92] and the photoelectric performance of ZnO was optimal when the number of shells reached three. pore channels of the porous heterozygous layer, thus reducing the likelihood sistance and electron-hole recombination. The S-scheme photocatalyst delive CH3OH formation rate of 81.1 µmol g −1 h −1 , which is roughly 40 and 20 times l that of bare ZnO (3.72 µmol g −1 h −1 ) and ZnO-ZnS (4.15 µmol g −1 h −1 ). On the oth porous ZnO@ZnSe core/shell nanosheet array material ( Figure 12A) was pre controlled manner [91]. The final n-type semiconductor composites had a prope CB band edge. In comparison to ZnO or ZnSe, more pairs of electron-holes we under visible light. Electrons tend to land on ZnO, which is aimed at methanol p Mei et al. prepared a ZnO microsphere with different numbers of shells [92] an toelectric performance of ZnO was optimal when the number of shells reached It is challenging to coat uniform 2D g-C3N4 nanofilm on the surface of 3D because of the difficulty in exfoliation process. Thus, Wang et al. [93] proposed static method and incorporated g-C3N4 nanofilm with porous ZnO nanospher a strong interaction. The heterojunction was then anchored on 3D graphen (GAs). The compound g-C3N4/ZnO/GA has extraordinary stability, maintain CO2 conversion rate, which is 92% of its original activity after 100 h.
ZnO/ZnS nanoflowers ( Figure 12B) were combined with g-C3N4 nanoshe 12C) to construct a double Z-scheme structure [94]. ZnO/ZnS nanoflowers prov specific surface area and g-C3N4 helps to absorb more photons under solar lig tion. Optimized interfacial charge transfer dynamics in ternary heterostructu characterized by photocurrent measurements. As a result, the formation rate o uct over the novel double Z-scheme mixture increases to 301 µmol g −1 h −1 on w ting. Hierarchical CuO/ZnO nanocomposites with p-n heterojunction were pre by the modified hydrothermal method. The photocatalysts are found to conve methanol in aqueous solution containing dimethylformamide (DMF) and trie (TEA) as electron donor under visible light irradiation. ZnO/NiO porous hollo with sheet-like subunits were obtained [96] by calcination of Ni-Zn MOFs. Nu n heterojunctions with n-type ZnO and p-type nickel monoxide were formulate ZnO/NiO. The porous hollow structure with the large specific surface area ca the absorption capacity of CO2 and light. It is challenging to coat uniform 2D g-C 3 N 4 nanofilm on the surface of 3D materials because of the difficulty in exfoliation process. Thus, Wang et al. [93] proposed an electrostatic method and incorporated g-C 3 N 4 nanofilm with porous ZnO nanospheres that has a strong interaction. The heterojunction was then anchored on 3D graphene aerogels (GAs). The compound g-C 3 N 4 /ZnO/GA has extraordinary stability, maintaining a high CO 2 conversion rate, which is 92% of its original activity after 100 h.
ZnO/ZnS nanoflowers ( Figure 12B) were combined with g-C 3 N 4 nanosheets ( Figure 12C) to construct a double Z-scheme structure [94]. ZnO/ZnS nanoflowers provide a large specific surface area and g-C 3 N 4 helps to absorb more photons under solar light irradiation. Optimized interfacial charge transfer dynamics in ternary heterostructure can be characterized by photocurrent measurements. As a result, the formation rate of H 2 product over the novel double Z-scheme mixture increases to 301 µmol g −1 h −1 on water splitting.
Hierarchical CuO/ZnO nanocomposites with p-n heterojunction were prepared [95] by the modified hydrothermal method. The photocatalysts are found to converse CO 2 to methanol in aqueous solution containing dimethylformamide (DMF) and triethylamine (TEA) as electron donor under visible light irradiation. ZnO/NiO porous hollow spheres with sheet-like subunits were obtained [96] by calcination of Ni-Zn MOFs. Numerous p-n heterojunctions with n-type ZnO and p-type nickel monoxide were formulated in mixed ZnO/NiO. The porous hollow structure with the large specific surface area can increase the absorption capacity of CO 2 and light.
Based on the vapor to solid mechanism, a novel ternary Ag/CeO 2 /ZnO nanocomposite [97] was synthesized by the facile thermal decomposition method. Oxygen vacancies introduced by line structure contributied to a narrow band gap of 2.66 eV, and this was further confirmed by DRS characterization. After the preparation of the ZnO/TiO 2 nano-tree arrays, Ag 2 S and ZnS were synthesized to modify the nano-tree arrays by cation exchange methods [98]. The core-shell structure of ZnO@ZnS prevented the decomposition of ZnO, and the modification of Ag 2 S reduced the Eg value of the composite and promoted the red-shifted of light absorption.

3DOM Structure Zn-Based Materials
Wang et al. [99] published metal-organic-framework-derived 3DOM N-C doped ZnO ( Figure 13) for efficient CO 2 reduction. The ultra-tiny CoO x clusters were anchored on the surface of catalyst and no Co-Co peak was found in CoO x /N-C-ZnO. The charge transfer rate was jacked up by ion doping and the recombination of electron-hole pairs was tamed because of the CoO x clusters. Furthermore, CoO x on the orderly connected channels can act as an electron trap to capture electrons, which makes a contribution to photoreaction efficiency. The density theory calculations (DFT) was also used to detect the CO 2 adsorption ability, and CoO x /N-C-ZnO exhibited the most negative CO 2 binding energy due to improved electron structure of adsorption site.
Molecules 2023, 28, x FOR PEER REVIEW tree arrays, Ag2S and ZnS were synthesized to modify the nano-tree arrays b change methods [98]. The core-shell structure of ZnO@ZnS prevented the dec of ZnO, and the modification of Ag2S reduced the Eg value of the composite an the red-shifted of light absorption.

3DOM Structure Zn-Based Materials
Wang et al. [99] published metal-organic-framework-derived 3DOM ZnO ( Figure 13) for efficient CO2 reduction. The ultra-tiny CoOx clusters wer on the surface of catalyst and no Co-Co peak was found in CoOx/N-C-ZnO. transfer rate was jacked up by ion doping and the recombination of electron was tamed because of the CoOx clusters. Furthermore, CoOx on the orderly channels can act as an electron trap to capture electrons, which makes a con photoreaction efficiency. The density theory calculations (DFT) was also use the CO2 adsorption ability, and CoOx/N-C-ZnO exhibited the most negative C energy due to improved electron structure of adsorption site.

Heterojunction
Recently, zeolitic framework (ZF) composite fabricated by the microwave-hydrothermal synthesis method (MWH) has attracted attention, which can provide a fast heating-speed and produce morphologically uniform samples. With biodegradable template, the zeolitic framework (ZF) was synthesized via MWH method from volcanic ashes. The NaAlSiO 4 (NAS) framework was composed of 50 nm circular channels and has a large surface area. Hip'olito et al. [18] embedded ZnO/CuO hybrid structure in the NAS channels, resulting in a ternary composite. The synergistic effect among ZnO, CuO and ZF support accelerated the photocatalytic process of water splitting and CO 2 reduction, which offers higher H 2 and HCOOH evolution rate.
The imidazole framework-8 (ZIF-8) molecular as a widely used zeolite (ZIF) composite, has an excellent competence of absorbing CO 2 , which is apt for CO 2 converting. Selective-breathing effect was wielded to boost CO 2 conversion efficiency through monolithic NF@ZnO/Au@ZIF-8 ( Figure 14A) catalyst [100]. Au particles were loaded on ZnO nanorods that grown on Ni foam (NF), the mixture was immersed in methylimidazole solution ( Figure 14B). Based on the results of electrochemical impedance spectroscopy (EIS) Nyquist plots, the alternating magnetic field was introduced to create magnetic heat, which leading to increased carrier density and improved photocatalytic performance. It is found that the selectivity of CH 4 cheeringly achieved 89% from 61% under photo-thermalmagnetic coupling effect.

Ion Doping
Many metal elements have been doped in ZnO to minish its band gap, such a Cu and Mn, among them Co distinguished itself because of the similar ion radius. In dition, the conversion of Co 3+ and Co 2+ results in oxygen vacancies, which greatly enha photocatalytic efficiency. Xie et al. [101] introduced Co 3+ with different mole ratio to microspheres precursor (including Zn 2+ , urea, and PVP), and the introduction of Co 3 not disrupt lattice structure as seen in the XRD model. It was revealed that the pres of both Co 2+ and Co 3+ in Co-ZnO from high-resolution spectrum, and Zn2p exhibited hi binding energy due to Zn 2+ charge transfer in 7% Co-ZnO. As the ratio of Co/Zn increa the conversion from Co 2+ to Co 3+ decreased, and the light response range graduall panded. The results showed that the electrochemical impedance of 7% Co-ZnO sam

Ion Doping
Many metal elements have been doped in ZnO to minish its band gap, such as Sb, Cu and Mn, among them Co distinguished itself because of the similar ion radius. In addition, the conversion of Co 3+ and Co 2+ results in oxygen vacancies, which greatly enhances photocatalytic efficiency. Xie et al. [101] introduced Co 3+ with different mole ratio to ZnO microspheres precursor (including Zn 2+ , urea, and PVP), and the introduction of Co 3+ did not disrupt lattice structure as seen in the XRD model. It was revealed that the presence of both Co 2+ and Co 3+ in Co-ZnO from high-resolution spectrum, and Zn 2p exhibited higher binding energy due to Zn 2+ charge transfer in 7% Co-ZnO. As the ratio of Co/Zn increased, the conversion from Co 2+ to Co 3+ decreased, and the light response range gradually expanded. The results showed that the electrochemical impedance of 7% Co-ZnO sample was the lowest band gap of 2.56 eV.

Summary
Great efforts have been made to design catalysts for CO 2 reduction on ZnO and the relevant research data are summarized in Table 3. Table 3. Summary of the recent results on the photoreduction of CO 2 by WO 3 -based materials.

Cu 2 O-Based Photocatalysts
Cuprous oxide (Cu 2 O) is a potential p-type semiconductor with a wide visible-light response range and high photo-electric conversion efficiency (18%) [106], and it displays attractive prospects in solar energy conversion and heterogeneous photocatalysis. Although Cu 2 O possesses many excellent properties, photocorrosion and the rapid recombination of e − /h + pairs affect its activity and limit its application. The photocorrosion is believed to occur in two ways: (1) self-reduction caused by generated electrons and (2) self-oxidation caused by the generated holes.
Self-reducing photocorrosion: Self-oxidative photocorrosion: Therefore, developing Cu-based catalysts with excellent activity, selectivity and stability has become the research hotspot in the area of the photocatalytic reduction of CO 2 . Many successful attempts have been made to improve the photostability and photocatalytic performance of Cu 2 O. In general, most studies focus on enhancing the charge transfer from Cu 2 O to reactants or cocatalysts to prevent charges from accumulating within the particles. A series of methods for improving the performance of Cu 2 O are discussed in detail in what follows.

Morphology Control
A branch-like Cd x Zn 1-x Se nanostructure was obtained [107] by the cation-exchange method, which was then mixed with Cu 2 O@Cu to form heterojunctions. Selenium (Se) vacancies were created during the ion exchange process and the crystal growth was lim-ited due to the additive diethylenetriamine (DETA), leading to insufficient coordination of the surface atoms, which then become active adsorption sites. Highly hierarchical branching-like structures assembled by one-dimensional structural materials not only facilitate electron accumulation at their tips but also increase the light-accepting area, and characterization results show that branching structures can effectively absorb visible light. Cd 0.7 Zn 0.3 Se/Cu 2 O@Cu step-scheme heterojunction exhibited a CO release yield of 50.5 µmol g −1 h −1 .
Ultrafine cuprous oxide U-Cu 2 O (<3 nm) was grown on the polymeric carbon nitride (PCN) (Figure 15) by the in situ method [108]. PCN has a narrow band gap of 2.7 eV that can capture visible light. Both ultrafine nanoclusters and Z-scheme heterojunction can protect U-Cu 2 O from degradation. The photocatalyst U-Cu 2 O-LTH@PCN has high stability, maintaining more than 95% activity after five cycles of testing, while bare Cu 2 O grades completely within three cycles. A large number of heterojunctions were formed by U-Cu 2 O particles and lamellar PCN, expediting the electron transfer efficiency. The product can convert CO 2 to methanol with water vapor under light irradiation at the high yield of 73.46 µmol g −1 h −1 . Ultrathin Ti 3 C 2 MXene with a high fraction of coordinated unsaturated surface sites was fabricated by Zhang et al. [109]. Via the HF etching method, different amounts of Cu 2 O were combined with Ti 3 C 2 nanosheets under the hydrothermal condition. The unique hexagram morphology of Cu 2 O, the 2D layer structure and excellent conductivity of Ti 3 C 2 T x nanosheets and the synergistic effect between the two composites promote the improvement of photoactivity. Zhang et al. reported the bifunctional catalyst of Cu 2 O@Fe 2 O 3 . Cu 2 O nanoparticles coated with an Fe 2 O 3 @carbon cloth electrode were used for both overall water splitting and CO 2 photoreduction [110].
Molecules 2023, 28, x FOR PEER REVIEW 20 promote the improvement of photoactivity. Zhang et al. reported the bifunctional cat of Cu2O@Fe2O3. Cu2O nanoparticles coated with an Fe2O3@carbon cloth electrode used for both overall water splitting and CO2 photoreduction [110].  [111] successfully achieved the morphology co of Cu2O nanocrystals by utilizing the selective surface stabilization of PVP on the plane of Cu2O. With different amounts of PVP, the surface area ratio of (111) to (100) subtly tuned, which resulted in the shape evolution of the system and various Cu2O s tures ( Figure 16). The detailed modification mechanism was elucidated from the s tural and kinetic perspectives. Figure 16. FESEM images of the Cu2O polyhedrons with different volume ratios of (100) to [111]. With different amounts of PVP, the surface area ratio of (111) to (100) was subtly tuned, which resulted in the shape evolution of the system and various Cu 2 O structures ( Figure 16). The detailed modification mechanism was elucidated from the structural and kinetic perspectives.

Preferentially Exposed Facets
Cu2O is an ideal compound to study the influence of electron-related effects. The rar occurrence of the O-Cu-O 180 o linear coordination of Cu2O makes its (111), (100) and (110 facets chemically active. Zhang et al. [111] successfully achieved the morphology contro of Cu2O nanocrystals by utilizing the selective surface stabilization of PVP on the (111 plane of Cu2O. With different amounts of PVP, the surface area ratio of (111) to (100) wa subtly tuned, which resulted in the shape evolution of the system and various Cu2O struc tures ( Figure 16). The detailed modification mechanism was elucidated from the struc tural and kinetic perspectives. Octahedral copper oxide that exposes the (111) crystal faces was decorated with low Fermi energy Ag nanoparticles. After coating with rGO, the ternary heterojunction cata Octahedral copper oxide that exposes the (111) crystal faces was decorated with low Fermi energy Ag nanoparticles. After coating with rGO, the ternary heterojunction catalyst [112] exhibits selective photocatalytic superiority towards CH 4 . The CO* radicals can be characterized via DRIFT spectra and DFT calculations, which is the key intermediate for the conversion of CO 2 to CH 4 . Two types of MoS 2 (p-type and n-type) and two shapes of Cu 2 O (cubic and octahedron) were synthesized and combined with each other [113], and the compositions possessed different electronic and structural properties. The heterostructures formed by the p-type MoS 2 with intrinsic conductivity had a higher photocatalytic activity, and the methanol production yield was as high as 76 µmol g −1 h −1 . Both Z-type and ii-type charge transfer mechanisms were built using an n-type MoS 2 mixture. For Cu 2 O, the cubes of the exposed (100) crystal plane with a higher binding affinity with MoS 2 transferred electrons more efficiently and produced methanol at a higher rate.
A 3D porous Cu was produced by electrodeposition method [114], being transformed into CuO 2 after following high-temperature annealing. Three-dimensional Cu 2 O delivers a 24-fold production of CO compared with the unremarkable and non-porous Cu. Additionally, more CO 2 accumulated and took reactions in the hollow space of 3D Cu 2 O to form C 2 products.

3DOM Structure Cu-Based Materials
The 3DOM Cu 2 O structure was luckily obtained [115] via polystyrene crystal templates. Under the contrived "sunlight" irradiation, incident light was reflected and absorbed around and around again. In the ultra-visible absorption spectra (350 to 800 nm), Cu 2 O with large orifices absorbs more photons than bulk samples, making it more advisable for solar applications. 3DOM Cu 2 O was prepared [116] by the electrochemical method to reduce CO 2 , and its Faraday efficiency was five times higher than that of Cu film. The CO 2 · − intermediate in 3DOM channels is more stable and leads to the possibility of forming CO and HCOOH products ( Figure 17). We look forward to the applications of inverse opal Cu 2 O in photocatalysis.
Molecules 2023, 28, x FOR PEER REVIEW 21 The 3DOM Cu2O structure was luckily obtained [115] via polystyrene crystal t plates. Under the contrived "sunlight" irradiation, incident light was reflected and sorbed around and around again. In the ultra-visible absorption spectra (350 to 800 n Cu2O with large orifices absorbs more photons than bulk samples, making it more ad able for solar applications. 3DOM Cu2O was prepared [116] by the electrochem method to reduce CO2, and its Faraday efficiency was five times higher than that o film. The CO2· − intermediate in 3DOM channels is more stable and leads to the possib of forming CO and HCOOH products ( Figure 17). We look forward to the application inverse opal Cu2O in photocatalysis. Figure 17. Proposed mechanism for CO2 reduction to CO and HCOOH on Cu2O-derived Cu (The symbol "*" represents the surface.) [116].

Heterojunction
Liu et al. [117] reported a facile solution and chemistry route to synthesize rGO corporated crystal Cu2O with various facets as visible-light-active photocatalysts for reduction. The enhanced activity was attributed to the formation of the heterojunction the existence of rGO as the electron transport mediator. M. Flores et al. [118] adopted microwave-hydrothermal method to couple the powders of Mg(OH)2, CuO and C The synthesis method allowed a sufficient interaction between Mg(OH)2/CuO and C without inhibiting the gas adsorption capacity of Mg(OH)2. They found that the pres of Cu2O favored the selectivity towards CH3OH production because a higher Cu + con tration led to better selectivity. Niwesh et al. [119] reported the formation of a p-n het junction between Cu2O and the SnS2/SnO2 nanocomposite that offered favorable reduc potentials and high stability, mainly owing to their intimate interfacial contact. In the sence of a sacrificial agent, the generation rate of NH4 + was 66.35 µmol g −1 h −1 Cu2O/SnS2/SnO2, which is 1.9-fold higher than that of SnS2/SnO2. However, the wor

Heterojunction
Liu et al. [117] reported a facile solution and chemistry route to synthesize rGOincorporated crystal Cu 2 O with various facets as visible-light-active photocatalysts for CO 2 reduction. The enhanced activity was attributed to the formation of the heterojunction and the existence of rGO as the electron transport mediator. M. Flores et al. [118] adopted the microwave-hydrothermal method to couple the powders of Mg(OH) 2 , CuO and Cu 2 O. The synthesis method allowed a sufficient interaction between Mg(OH) 2 /CuO and Cu 2 O without inhibiting the gas adsorption capacity of Mg(OH) 2 . They found that the presence of Cu 2 O favored the selectivity towards CH 3 OH production because a higher Cu + concentration led to better selectivity. Niwesh et al. [119] reported the formation of a p-n heterojunction between Cu 2 O and the SnS 2 /SnO 2 nanocomposite that offered favorable reductive potentials and high stability, mainly owing to their intimate interfacial contact. In the absence of a sacrificial agent, the generation rate of NH 4 + was 66.35 µmol g −1 h −1 for Cu 2 O/SnS 2 /SnO 2 , which is 1.9-fold higher than that of SnS 2 /SnO 2 . However, the work of Trang et al. also demonstrated the instability and photo-oxidation of Cu 2 O heterojunctions. Generally, most p-n heterojunctions are found to reduce the redox capacity of photogenerated charges. This is especially evident for Cu 2 O p-n type heterojunctions, as CuO is excessively formed on the surface of Cu 2 O under continued illumination, so there are recombination problems at the heterojunction interface. The construction of Z-scheme heterojunctions overcomes the limitations of p-n heterojunctions, namely the reduction in redox potential and charged carrier recombination at the p-n heterojunction interface. In a Z-type heterojunction, the redox potential can be maintained under the premise of high photo-induced electron transport rate. Zhang et al. [120] synthesized coal-based CNPs with an sp 2 carbon and multilayer graphene lattice structure, and loaded them onto the surface of Cu 2 O nanoparticles prepared by the in situ reduction of copper chloride. The rapid recombination of electron-hole pairs was suppressed by the introduction of CNPs. The energy gradient formed on the surface of Cu 2 O/CNPs facilitates the effective separation of electron-hole pairs for CO 2 reduction, improving the photocatalytic activity. Atomically dispersed In-Cu bimetallic catalysts were prepared [121] by the in situ pyrolysis method, in which carbon nitride acted as a carrier. The light-harvesting and charge separation efficiency were enhanced by regulating the loading amount of Cu and In, and the supreme generation rate of the photoreduction of CO 2 to ethanol reached 28.5 µmol g −1 h −1 with 92% selectivity. The DFT calculations showed that the introduction of an In atom in copper can accelerate electron transfer from carbon nitride to metal, improve the charge separation efficiency and increase the electron density of copper active sites. The presence of In-Cu sites exerted a synergistic effect, which could promote C-C coupling, lower the energy barrier of *COCO generation and increase ethanol yield. Zhao et al. [122] reported the indirect Z-scheme heterojunction of UiO-66-NH 2 /Cu 2 O/Cu, which achieved a high CO 2 photocatalytic conversion to CO. The SEM results of Cu 2 O, UiO-66-NH 2 and U/C/Cu-0.39 are shown in Figure 18A-C. In this catalytic system, UiO-66-NH 2 slowed down the photo-corrosion rate of Cu 2 O and increased the CO 2 adsorption capacity ( Figure 18D).

Summary
In conclusion, photochemical methods offer the opportunity to modulate the persistence and selectivity of Cu-based catalysts photoreduction to value-added compounds. The most recent photocatalytic CO2 reduction outcomes of Cu-based materials are listed in Table 4. Table 4. Summary of the recent results on the photoreduction of CO2 by Cu2O-based materials.

Summary
In conclusion, photochemical methods offer the opportunity to modulate the persistence and selectivity of Cu-based catalysts photoreduction to value-added compounds. The most recent photocatalytic CO 2 reduction outcomes of Cu-based materials are listed in Table 4.

CeO 2 -Based Photocatalysts
Cerium oxide (CeO 2 ) has an octahedral face-centered cubic fluorite structure, in which the coordination numbers of Ce and O are 8 and 4. When reduced at a high temperature, it can be converted to nonstoichiometric CeO 2−x (0 < x < 0.5). Notably, CeO 2−x maintains a fluorite crystal structure and forms oxygen vacancies after losing a certain amount of oxygen. CeO 2−x materials with different Ce/O ratios were also obtained in different conditions and it could be reconverted to CeO 2 again if it returned to an oxidizing environment. Because of the unique electrical structure, cerium oxide (CeO 2 ) is famous for the conversion sates between Ce 4+ and Ce 3+ , which have been studied as oxygen storage catalytic materials and solid oxide full cells by many scholars [124,125]. In summary, CeO 2 is a rare-earth metal oxide with a good photochemical stability, low cost and environment friendly characteristics. It is an important n-type semiconductor with a wide bandgap, and credible photocatalysts have been designed to reduce CO 2 and degrade pollutants [126].

Morphology Control
Yb-, Er-doped CeO 2 hollow nanotubes were synthesized [127] using silver nanowires coated with silica, and the products had a narrower band gap of 2.8 eV. The core-shell structured CeO 2 was converted into mesoporous hollow spheres by the Ostwald ripening method in the presence of urea and hydrogen peroxide [128]. CeO 2 nanocages can be fabricated by mixing (NH 4 ) 2 Ce(NO 3 ) 2 with templates of Cu 2 O nanocubes [129], in which Cu 2 O is finally sacrificed. The photocatalytic results [130] indicated that CeO 2 nanocages exhibit higher activity than hollow spheres.

Preferentially Exposed Facets
It was found that molecular CO 2 can be distorted and participate in reactions at a low energy on the CeO 2 surface [131]. A p-type NiO material was designed to modify the rod-like CeO 2 nanostructure [132], allowing electrons and holes to migrate to opposite directions. They then operated the Mott-Schottky test, which showed a typical p-n junction. The presence of hexagon-shaped NiO plates broadened the range of light responses, which can be verified in the UV-Vis absorption spectra. Graphene oxide (rGO) was introduced as a "network" of for photoreduction electron transportation ( Figure 19A-C). The impedance can be seen in the EIS Nyquist plot, which shows that the NiO/CeO 2 /rGO achieved the minimum value. The HCHO production rate of the ultimate catalyst was 421 µmol g −1 h −1 with the synergy of several favorable factors. It is worth mentioning that a range of in situ techniques have been used to detect oxygen vacancies, structural changes, free radicals and formate on the surface of CeO 2 .

Preferentially Exposed Facets
It was found that molecular CO2 can be distorted and participate in reactions at a low energy on the CeO2 surface [131]. A p-type NiO material was designed to modify the rodlike CeO2 nanostructure [132], allowing electrons and holes to migrate to opposite directions. They then operated the Mott-Schottky test, which showed a typical p-n junction. The presence of hexagon-shaped NiO plates broadened the range of light responses, which can be verified in the UV-Vis absorption spectra. Graphene oxide (rGO) was introduced as a "network" of for photoreduction electron transportation ( Figure 19A-C). The impedance can be seen in the EIS Nyquist plot, which shows that the NiO/CeO2/rGO achieved the minimum value. The HCHO production rate of the ultimate catalyst was 421 µmol g − 1 h −1 with the synergy of several favorable factors. It is worth mentioning that a range of in situ techniques have been used to detect oxygen vacancies, structural changes, free radicals and formate on the surface of CeO2. Figure 19. The structural diagrams of (A) n-CeO 2 nanorods, (B) p-NiO/CeO 2 composite and (C) NiO/CeO 2 /rGO hybrid composite [132].

Macroporous
Mesoporous N-doped CeO 2 (NMCe), a relatively ordered intermediate structure with enhanced CO 2 -capturing capability, was prepared without any convoluted procedures or expensive equipment [124]. In the Roman spectrum, the bands from 550 to 650 cm −1 , which are closely related to oxygen vacancy, were more salient than the MCe band. In addition, N-doped porous CeO 2 has a higher CO 2 absorption capacity than porous CeO 2 . All the above results conformed to the photoluminescence spectrum (PL) analysis, and the reduction gross yield of CO and CH 4 was 3.5 times higher than that of OMCe.
With the help of a suitable crosslinked and pyrogenic solvent, doped CeO 2 was uniformly fixed on transparent polymers by the in situ polymerization pathway [133]. By increasing the Ca/Ce molar ratio (wt.% < 20%), no peaks relating to CaO were observed in the p-XRD of samples and the original diffraction peak intensity became more strident due to the foreign ion's minuscule radius (Ca 2+ 0.1 nm, Ce 4+ 0.184 nm). The specific surface areas of three catalysts (CeO 2 , (20: 80) CaO/CeO 2 and CaO/CeO 2 NC-dispersed polymers as a whole) were 28.4, 58.3 and 224.7 m 2 g −1 , respectively, and heterojunction nanocomposites had the highest photocatalytic efficiency.

3DOM Structure Ce-Based Materials
Under the protection of poly alcohol, Zhang et al. [134] synthesized 3DOM CeO 2 that was loaded with Au-Pd alloys. 3DOM CeO 2 photocatalytic materials are expected to emerge in the field of CO 2 emission reduction, which could open up more possibilities for the development of super-catalysts.

Heterojunction
Researchers tried to combine CeO 2 with g-C 3 N 4 , which is popular for its energy bands and chemical stability. Through hard work, a three-dimensional porous g-C 3 N 4 (3DCN) was achieved, with the advantages of multi-channel structure. To accommodate more electrons and heighten the density of photoelectric currents, Zhao et al. [125] loaded Pt nanoparticles (5-6 nm) on CeO 2 /3DCN using photodeposition techniques that require UV lamp radiation. The photoreduction rate gradually increased as the CeO 2 amount rose in the range of 15~45% and the yield rates of 4.69 and 3.03 µmol·h −1 ·g −1 for CO and CH 4 were achieved, respectively, after decorating with Pt crystalline grains.
Under mild reaction conditions, carbon-doped hexagonal boron nitride (h-BN), known as boron carbon nitride (BCN), can reduce more carbon dioxide after dispersing cerium oxides on it. In the BCN/CeO 2 heterostructure [135], the N-O-Ce bond was formed by thermal precipitation method. It is of interest that the proportion of Ce 3+ /(Ce 3+ + Ce 4+ ) that influences electron transfer rate fluctuates with CeO 2 -loading amount, and the yield of CO peaked on 30%CeO 2 (selected from 10% to 70%). When exposed to UV light, CeO 2 crystals could absorb more photons than visible light, whereas h-BN does not exhibit absorption in the UV range. The establishment of heterojunction expanded the effective wavelengths and improved the absorption capability in ultraviolet and visible light. The •OH species was detected by ESR analysis to identify the type of the heterostructure and the result of no newly generated •OH species was in accord with the II-scheme system.

Summary
As a whole, the wider light harvesting range and longer separation time of charge pairs improved the photoreduction upshot. Overall, recent results on CO 2 photoreduction by CeO 2 -based materials are presented in Table 5.

Other 3DOM Materials
In order to introduce advanced porous structures to slow the self-aggregation of quantum dots, Wang et al. [143] devised 3DOM N-doped carbon (NC) to support CdS and ZnO QDs (Figure 20). They filled the interspace in an ordered PS microsphere template and then employed a pyrolytic treatment and in situ growth methods. Compared to bulky CdS, the 3DOM compounds have a larger cathodic current density and enhanced light harvesting, bearing a satisfactory carbon monoxide yield of 5210 µmol g −1 h −1 . A threedimensional SnO 2 inverse opal structure was synthesized as gas sensors, soot oxidation catalyst and photoanode. The 3DOM BiVO 4 /SnO 2 heterostructure was obtained by adding a BiVO 4 precursor to fill the space between SnO 2 skeleton and periodic PS template. The compatibility of energy states with SnO 2 significantly reduced their photoluminescence intensity. Meanwhile, Au nanoparticles enhanced the slow photon effect, which in turn increased the incident light utilization efficiency.
Moreover, some photoreduction results on 3DOM materials are listed in Table 6. From the above analysis, we can infer the great potential of macroporous materials for further development in the field of photocatalysis. three-dimensional SnO2 inverse opal structure was synthesized as gas sensors, soot oxidation catalyst and photoanode. The 3DOM BiVO4/SnO2 heterostructure was obtained by adding a BiVO4 precursor to fill the space between SnO2 skeleton and periodic PS template. The compatibility of energy states with SnO2 significantly reduced their photoluminescence intensity. Meanwhile, Au nanoparticles enhanced the slow photon effect, which in turn increased the incident light utilization efficiency. Proposed mechanism for the photocatalytic CO2 reduction on 3DOM CdS QD/NC. Schematic illustration of the photocatalytic CO2 reduction coupled with selective arylamine oxidation reaction system [143]. Proposed mechanism for the photocatalytic CO 2 reduction on 3DOM CdS QD/NC. Schematic illustration of the photocatalytic CO 2 reduction coupled with selective arylamine oxidation reaction system [143].

Conclusions
The photoconversion of CO 2 into solar fuels seems to curb greenhouse effect and resolve the energy crisis. In this review, the major research progresses of different metal oxide materials on solar-light-driven CO 2 conversion to fuels were carefully summarized. TiO 2 , WO 3 , ZnO, Cu 2 O and CeO 2 are the most common materials for photocatalysts among the numerous semiconductors. Even though considerable progress has been achieved, creating superb catalysts still presents several challenges. Researchers have presented treatment options to boost catalytic activity after apprehending the fundamental principles of objective reactions.
The main formulas for designing photocatalysts are as follows.

1.
Absorb more photons to produce excitons.

2.
Improve the migration efficiency of charge carriers.
Uplift the absorption capacity of CO 2.
In terms of material selection, it is wise to choose metal oxides as one of the active components because of their relatively suitable Eg and CB bands. From the perspective of morphology, compared with the solid and regular morphology, the structure of hollow ordered pores can promote the absorption of reactants and the desorption of products. A larger contact area between the catalyst and reactants and more active sites for the reaction can be provided. From the point of view of light absorption, the CB or VB positions can be adjusted after ion doping, which affects Eg and influences the position of spectral absorption; the slow photon effect in the 3DOM structure can also improve the light utilization. As for charge separation efficiency, it can be promoted at the interface of the heterojunction, and photonic crystals can also improve their separation efficiency by shortening the distance of charge movement to the interface. Numerous attempts have been made in the synthesis of metal oxide photocatalysts, which can yield CO, CH 4 , HCHO, HCOOH, CH 3 OH and C 2 H 5 OH. However, it remains a great challenge for current photocatalysts to satisfy actual industrial production demands. The efficiency and selectivity for target products cannot meet the requirements for industrial and commercial implementation.
To sum up, the migration-separation efficiency of photoinduced pairs is important to improve the catalytic activity. Scholars should concentrate on the multiple advantages of photonic crystals to design catalysts with better performance based on them. This review provided a wealth of experience and ideas for the exploitation of photocatalyst material selection, morphology control and active site design. There is still a considerable work to conduct in converting CO 2 from solar energy to fuel, and we believe more significant breakthroughs can be achieved regarding the efficiency, mechanism and durability of the photocatalyst.